Zinc phosphate conversion treatments and chromate treatments are currently in widespread use as underpaint coating treatments for various types of metals for the purpose of improving the post-painting corrosion resistance and paint film adherence. Zinc phosphate conversion treatments are mainly used when the metal substrate is composed of iron or is a composite of several types of materials, because it is difficult for chromate treatments to accommodate or be adapted to such substrates.
The crystals in the films afforded by zinc phosphate conversion treatment are subject to substantial variation in size as a function of the treatment conditions. Thick coatings of coarse crystals are ideal for plastic-working operations and rust prevention, but when such coatings are painted they may fail to provide a satisfactory paint film adherence. In fact, the most demanding uses of zinc phosphate conversion coatings as underpaint coatings require the generation of thin conversion films of uniform, dense, and fine coating crystals.
Two general methods are known for generating thin zinc phosphate conversion coatings. In one method, contact with the conversion treatment bath is suspended after only partial completion of the film-deposition reactions, thereby stopping these reactions. A problem with this method is that it gives an incomplete deposition of the conversion film and thus fails to provide complete coverage of the basis metal. As a result, post-conversion water rinse and drying processes can cause the substrate to rust, and the post-painting corrosion resistance will also be unsatisfactory.
The other general method relies on inducing fine-sized coating crystals. In this method the film-deposition reactions spontaneously terminate, or at least greatly slow, while the film is still thin, and the completed conversion coating completely covers the substrate. This method can provide both a satisfactory paint film adherence and post-painting corrosion resistance.
Immersion and spraying techniques are primarily used in the above described zinc phosphate conversion treatment technologies. In the absence of a prior "conditioning" treatment of the metal surfaces to be coated, immersion does not usually produce dense, fine coating crystals and requires a lengthy period of time for conversion treatment unless the treatment temperature is 55.degree. C. or greater. Spraying does produce a somewhat better fine-crystal formation in the coating than does immersion, but not usually enough to provide a fully satisfactory painting performance. And again, treatment temperatures of at least 55.degree. C. are required in order to complete the treatment in a relatively brief period of time.
A titanium colloid surface-conditioning treatment must usually be applied to the metal surface immediately prior to conversion treatment in order to obtain (a) fine-crystal formation in the coating and (b) a reduction in the treatment temperature to 50.degree. C. and below. This surface-conditioning treatment activates the surface of the metal substrate, with the result that, regardless of the use of immersion or spraying, the treatment temperature can be lowered, the treatment time can be shortened, and a fine-sized crystalline film that provides an entirely satisfactory painting performance can be obtained.
When metal surfaces are subjected to a zinc phosphate conversion treatment for the above-described purpose of providing an underpaint coating, the surface is ordinarily subjected to a titanium colloid surface-conditioning treatment immediately prior to the conversion treatment process. However, the titanium colloid dispersed in the surface-conditioning treatment bath aggregates with elapsed time after bath preparation, leading to a decline in the surface-conditioning activity with continued time of use. Japanese Patent Publication Number Sho 62-9190 9,190/1987! teaches management of the Mg/P.sub.2 O.sub.7 ratio in the surface-conditioning treatment bath in order to increase the stability of the titanium colloid, while Japanese Patent Application Laid Open Kokai or Unexamined! Number Sho 63-18084 18,084/1988! discloses the addition to the surface-conditioning treatment bath of an organic material as a stabilizer for the titanium colloid. Each of these methods, however, is less effective than would be desirable, with the result that in practice aged bath must be discharged and freshly prepared bath must be supplied on a continuous or at least frequent basis in order to cope with the decline in activity. This preparation and management of the surface-conditioning treatment bath is complex and labor intensive and entails a major economic burden due to its high reagent consumption. And of course, because treatment facilities are required in order to implement the surface-conditioning treatment, this raises costs for maintenance of the facilities and an expansion of the treatment space.
As a consequence of the various issues discussed above, there has recently been strong demand for the development of a surface treatment method that could omit the problematic titanium colloid surface-conditioning treatment while still being able to form on metal surfaces the uniform, fine, dense, and thin conversion films that are optimal as underpaint coatings.